In an image sensor, in many cases, analog pixel voltages are converted into digital signals, and digital signal processing is performed. When a pixel voltage is low, it is difficult to accurately convert the pixel voltage into a digital signal, and therefore, it is preferable to amplify the pixel voltage in advance. When the pixel voltage is low, the amplification factor is required to be set high, and when the pixel voltage is high, the amplification factor is required to be set low. Therefore, an amplifier of which amplification factor is variable is often used.
In this case, it is necessary to appropriately set the amplification factor. One of known setting methods includes applying various kinds of amplification factors to determine whether the output voltage from the amplifier is saturated or not, and selecting the maximum amplification factor at which the output voltage is not saturated.
However, in this method, the output voltage is saturated at least once. In such case, the pixel voltage needs to be read again, and it takes a long time to perform the amplification processing.